Oil shale formations contain vast quantities of valuable hydrocarbons. If 50% of the shale oil from the richest deposits in the Green River formation of Colorado, Utah, and Wyoming are recoverable, approximately 600 billion barrels of shale oil could be obtained. This is comparable to proved worldwide reserves of crude oil of about 650 billion barrels. Total shale oil resources of the world, both known resources and possible extensions, are estimated at 24,000 billion barrels. However an economical and environmentally acceptable method of producing these hydrocarbons is still being sought.
Mining of oil shale in order to retort the material at the earth surface is too costly, and furthermore it requires prohibitively large quantities of water, already scarce in the western United States, and in addition it leaves waste heaps of spent shale which are environmentally unacceptable. In fact the waste from retorting oil shale is 50% larger in volume than the extracted oil shale itself.
Efforts to produce shale oil in situ by various methods of heating the oil shale formation have been generally unsuccessful, primarily for two reasons. One reason is that oil shale is a very poor conductor of heat, with the result that it is practically impossible to raise the shale body to retorting temperature (at least 650.degree. F. and preferably 900.degree. F. to 1,000.degree. F.) by conventional means, where a source of heat is applied at one or more points and heat is conducted into the shale body therefrom. The other reason is that oil shale generally has little or no permeability, with the result that even such fluids as may be produced within the oil shale by pyrolysis are unable to find their way out of the shale body to a production well.
Methods have been proposed for opening up passages within the shale body in order to permit the transport of heat in the form of heated fluids into the oil shale and also to permit the removal from the shale of fluid products of pyrolysis. One such method, a modified in situ procedure, involves rubblizing the shale, by explosives for example, to produce many large flow paths for both heat and fluid transport. However, experience with rubblizing has been unsatisfactory because of severe channeling of gas flows through the rubblized bed, which means that the fluids used for driving product out of the shale bed move very unevenly through the bed, with some driving fluid breaking through to the production well while much of the driving fluid is still far back in the bed. Further, it has been found that because of great variablility in size of the chunks of shale after rubblizing there is correspondingly a great variability in their rate of heating up, with the result that the retorting process does not proceed, as is desirable, evenly along a front that moves through the shale bed. Such uneven advance of the front, with large chunks being pyrolyzed later than the small chunks, results in loss of overall efficiency of the process.
Another method, which is known as a true in situ procedure, involves creating fractures, such as by hydrofracturing, in the shale bed and using these fractures as the flow paths for the transport of both the pyrolyzing heat and the fluids resulting from pyrolysis. One such procedure involves the use of hot gases to supply the heat for pyrolysis. Hot gases may be provided in situ in the form of flue gas produced by burning some of the kerogen in the shale body. Burning of the kerogen is initiated by supplying air and a fuel, e.g. propane, to a fracture in the oil shale and igniting the fuel, which in turn initiates combustion in the kerogen. Continuation of the supply of air supports the continued combustion of the kerogen bordering the fracture. As this combustion proceeds through the oil shale from one end of the fracture to the other it is preceded by hot flue gas. The hot flue gas causes pyrolysis of the oil shale, and the combustion which follows the pyrolysis front is supported by the burning of the kerogen residue left after pyrolysis. This method works in principle but has met with little success because of the difficulty of creating a sufficient number of fractures with sufficient capacity for fluid flow.
Accordingly it is an object of the present invention to provide a new process in which, by a novel procedure of alternate fracture and production steps, a sufficient number of fractures with sufficient capacity for fluid flow are created in the oil shale bed to make it possible to achieve efficient, true in situ production of shale oil from oil shale.
Although the following description is expressed as a method for producing shale oil from oil shale, it is to be understood that the method is equally applicable to hydrocarbon-bearing formations other than oil shale, e.g. formations containing petroleum, heavy oils, tar sands, etc.